ElectroMechanical Systems include Micro ElectroMechanical Systems (MEMS) structures and Nano ElectroMechanical systems (NEMS) and MEMS and NEMS structures are used in a wide variety of applications including, for example, MEMS accelerometers in cars for airbag deployment or in consumer electronic devices such as game controllers, MEMS gyroscopes used in cars to detect yaw, optical switching, bio-MEMS applications, MEMS loudspeakers, inkjet printers and RF MEMS components such as antenna phase shifters. Advantages of using MEMS structures include they have mechanical properties but have small dimensions and can be manufactured using existing semiconductor processing technologies.
A MEMS transducer device, which may be used as an actuator or sensor, may include a movable structure fabricated on a semiconductor substrate including at least one mechanical stack comprising one or more mechanical layers of a material such as silicon or silicon nitride and at least one functional/actuating stack whose function is to facilitate the movement of the mechanical stack on actuation of the device. The actuating stack comprises one or more layers whose arrangement and function in relation to the mechanical stack depends on the type of MEMS transducer device. For example, in an electrostatic actuated device, the actuating stack comprises a stationary electrode that cooperates with a movable electrode formed on a mechanical layer to facilitate movement of the mechanical layer and movable electrode. In a magnetic actuated device, the functional stack comprises a magnetic layer which is arranged to apply an external force to the movable mechanical stack in cooperation with an external magnet. The actuating stack can also be a multi-layered stack including at least one actuating layer of a material such as a piezoelectric or a magnetic material formed over a mechanical stack such as a mechanical beam or cantilever. Due to its electromechanical conversion properties, lead zirconate titanate (Pb[ZrxTi1-x]O3 with 0<x<1) which is generally known as PZT, is the most commonly used piezoelectric material in MEMS devices. In the case of a piezoelectrically actuated MEMS switch device such as that shown in FIG. 1, the multi-layer movable structure includes an actuating layer comprising a PZT film 2 formed over a cantilever or beam 4 (which may be a silicon nitride or a silicon oxide cantilever) and electrodes 6 and 8 (which may be platinum electrodes) formed on either side of the PZT film 2 for applying a voltage across the PZT film. Contacts 10 and 12 provide the switch contacts of the device. As is well known, by applying appropriate voltages across the PZT film, the PZT film expands or contracts depending on the applied voltage by piezoelectricity which applies stress to the cantilever and results in the cantilever being deflected orthogonally (in a direction perpendicular to the stack) to open or close the MEMS switch device.
An article entitled ‘Design, fabrication and RF performances of two different types of piezoelectrically actuated Ohmic MEMS switches’ by Hee-Chul Lee, Jae-Hyoung Park, Jae-Yeong Park, Hyo-Jin Nam and Jong-Uk Bu in Journal of Micromechanics and Microengineering 15 (2005), pages 21098-2104, describes a piezoelectric actuated RF MEMS switch having a PZT capacitor formed on a cantilever.
US patent application no. 2005/0127792 describes a multi-layered piezoelectric switch for tunable electronic components comprising multiple piezoelectric layers, and metal layers alternated with the piezoelectric layers on a cantilever. Thus, this device uses stacked piezoelectric capacitors to form a piezoelectric actuated switch.
For MEMs transducer devices having a movable structure with at least one free end (for example, clamped with a single anchor) and being composed of multi-layer materials stacked together, the deflection of the movable structure can vary with temperature change due to the different values of the Coefficient of Thermal Expansion (CTE) for the different materials which form the movable structure, as in a bimetallic strip. This is especially true for piezoelectric actuated transducers. For example, for the piezoelectric actuated transducer of FIG. 1, the layers including the platinum (Pt) electrode 6, PZT film 2, and the platinum (Pt) electrode 8 of FIG. 1 will have a CTE of approximately 9.5 ppm/° C. compared to a CTE of 2-3 ppm/° C. of the silicon nitride cantilever 4. Thus, when the operation temperature changes, the Pt/PZT/Pt layers will expand (or contract) differently than the silicon nitride cantilever which results in changes in the transducers orthogonal deflection and thus, its performance. For example, for operation temperature changes over a 120° C. range, the piezoelectric MEMS switch device of FIG. 1 can experience a total deflection excursion of 7 μm. With large changes in the transducers deflection, the device may be made inoperable: for example, in the MEMS switch device of FIG. 1, the deflection due to temperature variations may cause the switch to be opened when it should be closed.
The same effect is seen in electrostatic switch devices having a movable structure with at least one free end and composed of a movable mechanical stack and a movable electrode layer formed on the movable mechanical stack as part of the functional stack. The difference of the CTE of the materials of the two layers can produce a thermal induced actuation.
The deflection described for a movable structure with at least one free end is due to the bending effect of a mechanical moment or force due to the multi-layer stack. The mechanical moment or force is typically referred to as the bending moment. This bending moment can have the same effect on other movable structures, such as, for example, clamped structures where the bending moment, due to a multilayer stack, is not present along the full structure. Such clamped structures include transducer devices having a movable structure (such as a mechanical layer or membrane) which is supported or clamped at ends of the movable structure and an actuating structure (such as a piezoelectric, electrostrictive or magnetostrictive actuating stack) located at the ends or at the centre of the movable structure. The actuating structure has a bending effect or induces a bending moment on the movable structure which causes the movable structure to move. As with the free end movable structures described above, the bending moment induced in such clamped structures may also vary with temperature variations.
It is known to provide thermal compensation in electrostatic switch devices by having additional layers which are identical and symmetrical to the movable electrode so as to compensate for the thermal behaviour of the movable structure.
For example, U.S. Pat. No. 6,746,891 describes a tri-layered beam MEMS switch device which is actuated by an electrostatic charge. When a voltage is applied across a stationary electrode on a substrate and an opposing movable electrode on a movable beam, an equal and opposite charge is generated on the stationary electrode and movable electrode. The charge distribution on the opposing electrodes produces an electrostatic force that is balanced by the elastic forces on the now deformed beam. As the voltage is increased, the charge increases in a non-uniform and non-linear fashion across the surface of the beam until a stability point is reached. The stability point is defined by the inability of the elastic forces to maintain equilibrium with the electrostatic forces and the beam snaps through to establish contact between two switch contact pads. This patent describes how an electrode interconnect is formed on the beam, which electrode interconnect is a structural match or structurally similar to the movable electrode so as to provide robustness to film stress and temperature induced beam deformation. In one embodiment, this patent teaches that the electrode interconnect is fabricated of the same material and dimensioned the same in order to provide mechanical balance. It is assumed that the stress is the same in the additional layer and the movable electrode.
For the electrostatic actuated device, only one (movable) electrode layer, combined with a stationary electrode layer, is required for the device to function. Therefore, it is not too complex to use a symmetrical tri-layered structure to realize the thermal/stress balance. For a more complicated device having multiple layers, such as piezoelectric actuated device, at least three layers (electrode/PZT/electrode) form the functional/actuating stack and a mechanical beam layer forms the mechanical stack. This makes the thermal balance more difficult to be met. Theoretically, the same symmetrical approach as used in the electrostatic actuated device can be used in an attempt to achieve thermal balance: that is, the same three layers can be deposited on the opposite side of the mechanical beam layer. In reality, however, this is complicated by manufacturing process variations. More layers mean more processing steps and larger variations, resulting in higher cost and less reproducibility. Also, the presence of the PZT layer before mechanical beam deposition may not be allowed due to serious contamination concerns.
In view of process constraints, it is not always possible to put the same material on both sides of the mechanical stack, due to serious contamination problems or because of process conditions. For example, a metallic layer used as a compensation layer for a metal electrode on top of a mechanical layer and made before the mechanical layer may not be compatible with the temperature deposition of the material of the mechanical layer. Thus, even for an electrostatic actuated device, there is a need to propose a solution to have improved thermal stability without using a symmetrical movable structure.
In addition to thermal stresses causing unwanted deformation or deflection of the movable beam in a MEMS device with variations in operating temperature, residual thin film stresses in the different layers of the multi-layered MEMS device can also cause unwanted deformation or deflection. Thin film stresses arise from the deposition processes used to produce the layers of the multi-layered device.
For example, in the case of the piezoelectrically actuated MEMS switch device, such as that shown in FIG. 1, having an actuating structure comprising Pt/PZT/Pt layers formed over a silicon nitride beam 4, the PZT film 2 needs to be annealed for crystallization. As a result, the stresses of the Pt/PZT/Pt actuating layers are substantially controlled by the 600-700° C. anneal temperature which ensures good piezoelectric properties. For a silicon nitride beam 4 formed by Low Pressure Chemical Vapor Deposition (LPCVD), there is a limited range of stress attainability (300 MPa-1100 MPa) for the beam. Thus, provided the stress of the beam which is required to achieve stress balance falls within this range then, stress balance can be achieved. However, it is not always possible to achieve stress balance between the actuating layers and the beam in the vertical direction so as to realise a flat beam (i.e. zero deflection or deformation in the vertical direction). Residual stress can result in a deflection of micrometers or tens of micrometers at the tip of the beam. Achieving stress balance is more difficult with manufacturing steps requiring extreme temperatures.
An article entitled ‘Mitigation of residual film stress deformation in multilayer microelectromechanical systems cantilever devices’ by Jeffrey S. Pulskamp, Alma Wickenden, Ronald Polcawich, Brett Piekarski and Madan Dubey, in J. Vac. Sci. Technol. B 21(6), November/December 2003, pages 2482-2486 describes an approach to compensate for the residual thin film stress deformation in MEMS devices based upon analytical and numerical modelling and in-process thin film characterization. Using the equations which result from the modelling and detailed knowledge of the fabrication processes employed, the later deposition steps which possess the greater degrees of process control are used to form layers to compensate for the process variability in layer stress and thickness in the earlier processed films.
An article entitled ‘Fabrication of PZT actuated cantilevers on silicon-on-insulator wafers for a RF microswitch’ by Hong Wen Jiang, Paul Kirby and Qi Zhang in Micromachining and Microfabrication Process Technology VIII, San Jose Calif., USA 2003, pages 165 to 173, describes a processing scheme for fabricating PZT actuated silicon cantilevers using silicon-on-insulator wafers. A PZT actuating layer comprising a bottom titanium/platinum electrode, a PZT layer and a top Titanium/Gold electrode is formed on a silicon beam. A silicon dioxide interface layer is formed between the PZT actuating layer and the beam. Stress balancing is addressed during design of the device by determining the stress state of the layers that form the cantilever and choosing by calculation and appropriate adjustments in the deposition processes, the stress in the layers so as to attempt to balance stress across the cantilever and to obtain relatively flat cantilevers.
Even a basic MEMs device using a symmetric movable structure may not be stress balanced if the layers below and above the mechanical layer have different residual stresses due to different deposition conditions.
However, stress balancing during design is limited to the accuracy with which the mechanical parameters of the different layers can be measured and to the stability of the manufacturing processes.